Method and apparatus for induction heating of metal strips



iNVENTORS WITNESSES: A f

am Y E H w m 4 W N 2 m mx e y 3 6 Patented Aug. 31, 1948 METHOD AND APPARATUS FOR INDUCTION HEATING OF METAL STRIPS Robert M. Baker, Pittsburgh, and Gaylord W.

Penney, Wilkinsburg, Pm,

assignors to Westinghouse Electric Corporation, East. Pittsburgh, Pa, a corporation of Pennsylvania Application September 8, 1944. Serial No. 553,380

2 Claims. 1

This invention is directed to providing low cost induction heating of travelling metallic materials such as strips, sheets, plates and the like for inductively heating the material from a state in which it is ferromagnetic to a state in which it is paramagnetic, using the word paramagnetic in its more modern sense as applicable to a, material having a magnetic permeability about that of free space, but not less.

It is known that materials such as nickel, iron with and without cobalt, and similar materials are usually magnetically relatively highly permeable at ordinary temperatures, although to different extents. As the temperature is raised, the magnetic permeability may decrease, but at any rate there is a fairly definite temperature, called the Curie point, at which the material ceases to be ferromagnetic and becomes substantially the same as that of other non-ferrous materials insofar as magnetic permeability is concerned. In other words, the ferrous materials are comparatively magnetic or ferromagnetic, below the Curie point, and lose this property above it. For nickel the Curie point is approximately 370 0., for steel about 730 to 780 C. and for 35% cobalt, 65% iron about 950 C.

Ferromagnetic material can be heated by inducing eddy currents therein through the action of an inter-linking magnetic field of suitable frequency. In the case of elongated flat material, the magnetic field may be directed along the length-direction of the material or the magnetic field may be directed through the material in a direction transverse to its length. In the former, the heating is by magnetic lines substantially parallel to the length of the material, and is designated as longitudinal flux induction heating. In the latter, the heating is by magnetic lines directed transverse to the length-direction of the material, and is called transverse flux induction heating. An illustration and elucidation of longitudinal flux may be found in the patent application of G. E. Stoliz and R. M, Baker, Serial No. 464,040 (now abandoned). filed October 31, 1942, and of transverse flux in the patent appication of R. M. Baker, Serial No. 521,229, filed February 5, 1944, and the various patent applications mentioned therein, all of which claim various aspects of such induction heating.

Ferromagnetic strip can be easily heated to the Curie point with a longitudinal flux of suihciently high frequency, but as the thickness of the strip is decreased, the frequencies must be increased. With longitudinal flux, frequencies in the order of several megacycles must be used to heat strip only 2 a few mils thick above the Curie point. In th case of travelling strip, practical heat-treatment on a large scale requires such high powers as to make the use of such high frequencies unsatisfactory and undesirable because of low efliciencies of power conversion and transfer, and the dangerous voltages involved.

It is an object of our invention is provide an induction furnace means which will heat-treat elongated materials of the type described at a minimum cost in apparatus and power consumption.

It is a further object to provide a novel means for inductively heat-treating travelling materials of a type described, for raising the material from temperatures at which it is ferromagnetic to temperatures above that at which it become paramagnetic.

Other obiects, features, methods and innovations of our invention will be apparent from the following description thereof, which is to be taken in connection with the accompanying schematic drawing in which:

Figure 1 is a view of a system for inductively heating ferromagnetic strip in accordance with our invention, and

Fig. 2 is a transverse view, partly in section, of part of Fig. 1.

In accordance with our invention, a magnetic strip 2 is pulled by means of any suitable stripmoving equipment comprising rollers 4 which provides a predetermined path along which the strip is pulled. In this path are successively arranged a relatively wide helically-wound longitudinal flux induction heating means comprising v coil means 6 through which the strip axially passes, and a transverse flux induction heating means comprising a pair of spaced pole or core structures 8 and in through the air-gap [2 of which the strip passes after having passed through the coil means 6.

The coil means 6 may be energized from a tubeo -cillator generator ['4 which includes a resonant circuit 16 or circuits for determ ning the frequencies at which it operates. The tube oscillator i4 derives a variable high voltage directcurrent operating power from a power line i'l through the medium of a variable induction regulator it which controls the voltage on a transformer 20 fed by the power line H. The output of the transformer 20 is rectified by rectiflers 22 and delivered to the tube-oscillator generator. Both the frequency of oscillation of the tube oscillator and the power delivered to the coil means 8 can be varied or regulated.

The core structures 8 and I include magnetizin coils 24 which derive their power from an electrodynamic generator 20. A phase improving means comprising a capacitor 28 may be included in this energizing circuit.

When the strip moves in the direction of .the arrows 30, it first passes centrally through the coil means 6 which provides a longitudinal flux of suitable energy and frequency for heating the strip to about the Curie point. While the hot strip is still at or near the Curie point it passes into the air-gap l2 between the core-structures 8 and III for further heating by the transverse flux across the gap.

It is preferable to heat strip by using longitudinalflux because the heating is generally inherently satisfactoril uniform across the width of the strip. No additional equipment is required to control the temperature distribution across the width of the strip. However, longitudinal flux induction heating has the practical limitation that 4 I t is the thickness or the strip in centimeters; r is the average electrical resistivity of the strip in ohm-centimeters; and f is .the frequency in cycles per second.

Up to the Curie point the permeability of ferro-' magnetic strip is high, and high saturation flux densities are easily obtained. However, with all other factors remaining the same. at the Curie point the permeability drops to practically unity and the power generated in the strip drops at a considerably faster rate than the permeability.

- For ordinary 10 mil steel strip having an average resistivity 01 45 X 10- ohm-centimeters and a surface-permeability of 120, when cold, the power delivered drops in a ratio of as much as 9000 to l. The result is that the strip cannot be heated materially above the Curie point by the same apparatus, without inordinate and undesirable the strip cannot be heated much above the Curie point unless a very large Jump in frequency to the order of magacycles is made, and extremely low power factor or rate of conversion to heating power can be tolerated. Even then the voltage across a coil means for inducing the heating required for large-scale production may be of dangerous magnitudes.

On the other hand, transverse flux induction heating can heat strip with power supplies of frequencies in the order of that obtainable from commercial power lines and rotary induction altemators. An upper limit of about 15,000 cycles per second is the maximum frequency for which such machines are at present designed for high power use.

In general, the wattage required for heating travelling strip is determined by the volume of strip to be treated per second, the specific heat of the strip, and the desired temperature rise.

.This volume, of course, depends on the speed at which the, strip moves. Thin travelling strip cannot be heated from ordinary temperatures to temperatures much above the Curie point by longitudinal flux alone, and such heating is not practicable by transverse flux alone. By combining the longitudinal flux induction heating with the transverse flux induction heating it is possible to so heat the strip economically and safely and with apparatus of minimum cost. The longitudinal flux induction heating means supplies enough power to raise the strip about up to the Curie point, and the transverse flux induction heating means substantially immediately thereafter supplies the power for heating the strip to higher temperatures.

Considering longitudinal flux heating, and assuming that the heating is under most emcient conditions so that the depth of penetration is close to one-half the thickness of the travelling strip, the power generated in the strip by the longitudinal flux produced by the heating coil means 6, toward supplying the wattage required for heating the travelling strip, can be expressed by the formula where Wa is the power in watts put into a square centiincreases in frequency.

Equation 1 can be rewritten as flux density at the Curie point is so abrupt and extreme that the required compensating adjustments in frequency are outside satisfactory operation of commercial tube-oscillators of high power.

As to decreasing the frequency, it becomes theoretically and practically impossible to put any-significant wattage into the strip at frequen-- cies below about or .6 times the optimum frequency value.

As an example of longitudinal flux heating with a predetermined heating coil means of relatively short span so that heat radiation from the strip can be ignored because it is such a small proportion of the heatsupplied to the strip, assume that a strip requires a wattage of 70 watts per square centimeter for heating to the Curie point. On this basis, 20 mil steel strip having an average resistivity of about 45 10 ohm-centimeters cannot be conveniently heated to the Curie point with frequencies less than about 17 kc. Ten mil strip requires about 48 kc., and 5 mil strip about kc. For heating above the Curie point frequencies in the order of 5500, 22,000, 88,000 kc., would be required for heating the 20 mil, 10 mil, and 5 mil strip, respectively, having an average resistivity of twice that below the Curie point.

In heating a. non-magnetic strip by transverse flux provided by a pair of core structures, each comprising a plurality of poles separated by a plurality of slots, 'with facing poles of opposite polarity, the frequency f, of the source 26 or the equivalent, in cycles per second, should be such that where g is the width of the air-gap between core structures in centimeters; and p is the pole-pitch in centimeters; the pole pitch p being more than three times the air-gap g.

In view of the foregoing, it is obvious that we have provided an induction heating system in which strip and similar magnetic material can be heated in a continuous production line to treatment temperatures well above the Curie P int.

We claim as our invention:

1. A method of inductively heating i'erromag-.

netic material, which comprises moving the material longitudinally along a predetermined path, heating the material as it passes through a part of said path, through the medium of an alternating longitudinal magnetic field pulsating at frequencies in the range provided by a tube-oscillator generator, to a temperature at which the material is paramagnetic, and then, while the material is still paramagnetic, heating the material across its width as it passes through a subsequent part of said path, through the medium of an alternating transverse magnetic field pulsating at frequencies in the range provided by a rotary alternator, to a temperature above the first said temperature.

2. Inductive heating means for heating material, comprising means providing a work-passage through which material to be inductively heated may be passed, said means comprising a plurality oi. inductive heating means, one of said inductive heating means comprising an inductor coil means for providing a longitudinal magnetic field for said work-passage, another or said inductive REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,377,574 Frary May 10, 1921 1,412,484 Mordey Apr. 11, 1922 1,900,842 Northrup Mar. 7, 1933 1,912,214 Northrup May 30, 1933 2,371,459 Mittelmann Mar. 13, 1945 2,381,323 Vore Aug. 7, 1945 2,401,899 Bierwirth et a1 June 11, 1946 

